The present invention relates to a holder for a reflecting member and also to an exposure apparatus having the holder. More particularly, the present invention relates to a holder which holds a reflecting member of high accuracy such as a reflecting member provided in an optical system that projects a mask pattern onto a photosensitive substrate in a photolithography process to produce, for example, semiconductor devices, image pickup devices (e.g. CCDs), liquid-crystal display devices, or thin-film magnetic heads. The present invention also relates to an exposure apparatus having the holder.
In a photolithography process (i.e. a process for forming a resist image of a mask pattern on a substrate) for producing, for example, semiconductor devices, a projection exposure apparatus (e.g. a stepper) is used in which a pattern of a reticle as a mask is transferred onto a photoresist-coated substrate (or a wafer or the like) through a projection optical system. Examples of projection exposure apparatus used for this purpose include static exposure apparatuses, e.g. step-and-repeat type reduction projection exposure apparatus (so-called stepper), and scanning exposure apparatuses, e.g. step-and-scan or slit-scan exposure apparatus.
In a typical projection exposure apparatus, a pattern written on a reticle is transferred onto a substrate as an image reduced to from 1/5 to 1/4 through a projection optical system. Semiconductor devices and so forth are demanded to be as small as possible with a view to reducing the size of equipment incorporating them and also to lowering the power consumption. However, to minimize the size of semiconductor devices and so forth, patterns to be transferred onto substrates must be made finer. In recent years, in particular, the scale of integration of ULSI has further increased, and it has been demanded to form a pattern having a linewidth of 0.35 micron or less, for example, on a substrate.
Conventional projection exposure apparatuses use the g-line (wavelength: 436 nanometers), the i-line (wavelength: 365 nanometers), etc. as exposure light. Recently, however, KrF excimer laser light (wavelength: 248 nanometers), ArF excimer laser light (wavelength: 193 nanometers), etc. have come into use in order to form a pattern with a narrow linewidth as stated above on a substrate. Incidentally, when an exposure process is carried out by using exposure light in such a wavelength band, a catadioptric system using a reflecting mirror is frequently employed as a projection optical system because it has the advantage that a satisfactory demagnification ratio can be obtained and the optical system per se can be made compact.
However, to transfer a pattern with a narrow linewidth as stated above onto a substrate, it is necessary not only to change the type of exposure light but also to improve the catadioptric system in the accuracy concerning the configuration, for example, and to bring its optical characteristics close to the ideal values. In general, such a catadioptric system is formed from a plurality of optical elements. Accordingly, it is necessary in order to improve the overall accuracy of the catadioptric system to improve the accuracy of each individual optical element. A concave mirror or a plane mirror as a reflecting optical member that constitutes a part of the catadioptric system exhibits extremely high accuracy in the state of being a single element. However, when the degree of accuracy required for the optical system becomes extremely high, some problems arise. That is, the reflecting surface is deformed on account of the way in which it is supported, and this causes the optical performance to be degraded. This problem will be explained below.
A concave mirror is produced by forming a concave surface of a predetermined configuration on a glass material, a ceramic material, or Zerodur (registered trademark) and evaporating aluminum coating or the like on the concave surface. Glass and other similar material are hard by nature. Therefore, in ordinary optical equipment, even if the reflecting mirror is deformed owing to the way in which it is supported, it is possible to ignore degradation of optical characteristics caused by the deformation of the reflecting mirror.
However, in the recent projection exposure apparatuses, a catadioptric system of extremely high accuracy is needed as stated above; therefore, only a slight deflection of the reflecting mirror due to its own weight may cause a problem. Accordingly, even when the concave surface is machined with high accuracy, if the way of supporting the reflecting mirror as incorporated in optical equipment differs from that during the machining process, the concave surface may be deformed, resulting in a concave mirror incapable of exhibiting the desired optical characteristics when used. Accordingly, to produce a concave mirror of high accuracy, such a technique is often adopted that a concave surface is formed on a stock in a state where the surface being machined is allowed to deflect by its own weight by supporting the stock in a position in which the resulting concave mirror will be actually supported, and then carrying out vapor deposition.
The concave mirror produced by the above-described technique exhibits the highest optical performance when the way of supporting the concave mirror during machining is the same as the way of supporting the concave mirror as incorporated in optical equipment. However, it is difficult to make the mirror supporting modes on the two occasions completely agree with each-other. Accordingly, the way of supporting the concave mirror during machining and the way of supporting the concave mirror as incorporated in optical equipment are made as similar to each other as possible, thereby making deformations on the two occasions similar to each other and thus obtaining the desired optical performance. The similarly in deformation of the concave surface between the concave mirror during machining and the concave mirror as incorporated in optical equipment will hereinafter be referred to as "reproducibility of deflection".
FIGS. 17(a) and 17(b) are diagrams showing a concave mirror as supported by a support frame according to a prior art. FIG. 17(a) is a top plan view of the concave mirror supported by the support frame. FIG. 17(b) shows the concave mirror and the support frame in a sectional view taken along the line XVIIB--XVIIB in FIG. 17(a). It should be noted that the concave surface of the concave mirror Ml is machined in a state where the concave mirror Ml is supported by a support frame having the same configuration as that of the support frame 540 in an exposure apparatus.
As shown in FIGS. 17(a) and 17(b), a cylindrical support frame 540 has a small inner-diameter portion 540a and a large inner-diameter portion 540b, which are in a coaxial relation to each other. The large inner-diameter portion 540b has a peripheral groove 540c formed on the upper end thereof. The outer diameter of the concave mirror M1 is larger than the inner diameter of the small inner-diameter portion 540a and equal to the inner diameter of the large inner-diameter portion 540b. Accordingly, when the concave mirror Ml is inserted into the large inner-diameter portion 540b of the support frame 540, the outer peripheral portion at the back of the concave mirror M1 is supported by the upper surface of the small inner-diameter portion 540a. It should be noted that the concave mirror M1 is secured to the support frame 540 by a silicone adhesive filled in the peripheral groove 540c.
The outer peripheral portion at the back of the concave mirror M1 contacts the upper surface of the small inner-diameter portion 540a at the entire circumference thereof in theory, but the concave mirror M1 is partially supported by the support frame 540 at two or three points in practice because the respective contact surfaces of the concave mirror M1 and the support frame 540 cannot be finished with perfect flatness and parallelism.
Under such circumstances, it is impossible in designing to predict at which points the outer peripheral portion at the back of the concave mirror M1 will be supported. Therefore, the reproducibility of deflection cannot be obtained. In other words, no matter how accurately the concave surface is machined when the concave mirror M1 is in the state of being a single element, when the concave mirror M1 is supported by the support frame 540, the concave surface may be deformed in various ways according to points at which it is supported. Therefore, it may be impossible to obtain the desired optical characteristics. It should be noted that if the concave mirror M1 is secured to the support frame 540 by using bolts or the like, the degree of adhesion between the two members increases. However, the fastening force applied through the bolts may cause the concave surface to be distorted even more unfavorably.
FIGS. 18(a) and 18(b) are diagrams showing a concave mirror as supported by a support frame according to another prior art which has a configuration different from that of the support frame shown in FIGS. 17(a) and 17(b). FIG. 18(a) is a top plan view of the concave mirror supported by the support frame. FIG. 18(b) shows the concave mirror and the support frame in a sectional view taken along the line XVIIIB--XVIIIB in FIG. 18(a). It should be noted that the concave surface of the concave mirror M1 is machined in a state where the concave mirror M1 is supported by a support frame having the same configuration as that of the support frame 640 in an exposure apparatus.
The prior art shown in FIGS. 18(a) and 18(b) differs from the prior art shown in FIGS. 17(a) and 17(b) in that the cylindrical support frame 640 has a uniform inner diameter over the entire length thereof and further has three projections 640a inwardly projecting from a central portion of the support frame 640 at equal angular intervals. The projections 640a support the outer peripheral portion at the back of the concave mirror M1.
According to the prior art shown in FIG. 18(a) and 18(b), the outer peripheral portion at the back of the concave mirror M1 is supported at three points set at approximately equal intervals. Consequently, the supporting structure provides an improvement in the reproducibility of deflection of the concave surface over such a supporting structure that the concave mirror M1 is supported by the support frame 540 shown in FIGS. 17(a) and 17(b). However, even within the narrow upper surface of each projection 640a, there may be undesired partial contact between the projection 640a and the back surface of the concave mirror M1. Such partial contact causes the reproducibility of deflection to be degraded and thus makes it difficult to form a concave mirror of high accuracy.
Supporting the concave mirror M1 by three-point seats means that the concave mirror M1 is not supported by any other than the three-point seats. Accordingly, the peripheral portion of the concave mirror M1 is deflected in a trefoil shape (i.e. non-supported portions lower) by the weight of the concave mirror M1 (by the action of gravity G). Consequently, the concave mirror M1 is distorted asymmetrically with respect to the optical axis. In this case also, the optical performance is deteriorated.
A plane mirror is also generally produced by evaporating aluminum coating or the like on one surface of a flat glass, ceramic or Zerodur (registered trademark) plate. Incidentally, because glass and other similar materials are hard by nature, deformation of the plane mirror caused by supporting it is only slight. Therefore, in ordinary optical equipment, it is possible to ignore degradation of optical characteristics caused by the deformation of the plane mirror no matter how it is supported.
However, in the recent projection exposure apparatuses, a catadioptric system of extremely high accuracy is needed as stated above. Therefore, the overall optical characteristics of the catadioptric system may be degraded by distortion of the plane mirror that occurs depending upon the way of supporting the plane mirror. The similarly in deformation between the plane mirror during machining and the plane mirror as incorporated in optical equipment will hereinafter be referred to as "reproducibility of deflection".
FIGS. 19(a) and 19(b) are diagrams showing a plane mirror as supported by a support frame according to a prior art. FIG. 19(a) is a top plan view of the plane mirror supported by the support frame. FIG. 19(b) shows the plane mirror and the support frame in a sectional view taken along the line XIXB--XIXB in FIG. 19(a).
In FIGS. 19(a) and 19(b), a prism-shaped support frame 710 whose section perpendicular to the longitudinal direction has an isosceles triangle-shaped configuration has a horizontal surface 710a, a vertical surface 710b perpendicular to the horizontal surface 710a, and a slant surface 710c extending at an angle to each of the horizontal and vertical surfaces 710a and 710b. The support frame 710 is provided with an opening 710d extending from the horizontal surface 710a to the slant surface 710c and also provided with an opening 710e extending from the vertical surface 710b to the slant surface 710c. The openings 710d and 710e have a mutual opening end 710f at the slant surface 710c.
A plate-shaped plane mirror M52 has a flat mirror surface M52a placed in contact with the slant surface 710c so as to face the opening end 710f. A back surface M52b reverse to the mirror surface M52a is supported by four leaf springs 711. Each leaf spring 711 is secured to the support frame 710 by using a bolt 712. Accordingly, the plane mirror M52 is pressed toward the support frame 710 by resilient forces from the leaf springs 711.
Thus, the plane mirror M52 is pressed toward the slant surface 710c, and hence the degree of adhesion between them increases. However, because the slant surface 710c is provided with the opening end 710f, counterforce that the plane mirror M52 receives from the slant surface 710c is not uniform over the whole surface of the plane mirror M52. Therefore, even if the mirror surface M52a has a high degree of flatness when the plane mirror M52 is a single element, the flatness may be degraded by placing the plane mirror M52 in close contact with the support frame 710. The degradation of the flatness of the mirror surface M52a causes a reduction in optical characteristics, e.g. image-formation characteristics, of a catadioptric system using the plane mirror M52.